A system accident (or normal accident) is an "unanticipated interaction of multiple failures" in a complex system. This complexity can either be of technology or of human organizations, and is frequently both. A system accident can be easy to see in hindsight, but extremely difficult in foresight because there are simply too many action pathways to seriously consider all of them. Charles Perrow first developed these ideas in the mid-1980s. William Langewiesche in the late 1990s wrote, "the control and operation of some of the riskiest technologies require organizations so complex that serious failures are virtually guaranteed to occur."

Safety systems themselves are sometimes the added complexity which leads to this type of accident. Maintenance problems are common with redundant systems. Maintenance crews can fail to restore a redundant system to active status. They are often overworked or maintenance is deferred due to budget cuts, because managers know that they system will continue to operate without fixing the backup system.